1. Field of the Invention
The present invention relates to a low-pass filter, and more particularly to a low pass-filter capable of being integrated in the integrated circuit and reduce the prime cost.
2. Description of the Prior Art
Power supplies are widely applied to provide regulated voltage and current for various electronic products. To improve the stability of the power supplies, compensation capacitors are usually coupled to the control integrated circuit (controller chip) of the power supplies externally. Thus, extra pins must be added to the controller chip for coupling to the external compensation capacitors, which increases the packaging cost of the controller chip. Furthermore, since the external compensation capacitors are usually bulk capacitors, they will occupy considerable layout space if they are integrated within the controller chip. Detailed explanation is given below to describe disadvantages of the above. Referring to FIG. 1, it shows a conventional power supply disclosed in U.S. Pat. No. 7,362,592 (Yang). The conventional power supply comprises a transformer 10 having a primary winding NP, a secondary winding NS, and an auxiliary winding NA. A terminal of the primary winding NP is coupled to an input voltage VIN. A transistor 20 is coupled to another terminal of the primary winding NP and a ground reference via a resistor 30. A controller 70 has a supply terminal VCC, a ground terminal GND, a detection terminal DET, a sense terminal CS, a pulse width modulation terminal VPWM, a voltage-compensation terminal COMV, and a current-compensation terminal COMI. The pulse width modulation terminal VPWM of the controller 70 generates a switching signal VPWM to control the transistor 20 for switching the transformer 10 and regulating the output voltage VO and/or the output current IO of the power supply.
A terminal of the secondary winding NS is coupled to a rectifier 40. Two terminals of a filter capacitor 45 are respectively coupled to the rectifier 40 and another terminal of the secondary winding NS. The transformer 10 stores energy when the transistor 20 is turned on. Once the transistor 20 is turned off, the energy stored in the transformer 10 is released to an output terminal of the power supply via the secondary winding NS, and a secondary-side switching current IS, an output current IO and an output voltage VO are generated accordingly. Meanwhile, a reflected voltage VAUX is generated at the auxiliary winding NA of the transformer 10. The reflected voltage VAUX charges a capacitor 65 via a rectifier 60. The capacitor 65 is coupled to the supply terminal VCC of the controller 70 to power the controller 70.
The detection terminal DET of the controller 70 is coupled to a joint of the resistors 50 and 51 of a voltage divider. A terminal of the resistor 50 is coupled to the auxiliary winding NA. A terminal of the resistor 51 is coupled to the ground reference and the ground terminal GND of the controller 70. The detection terminal DET is coupled to receive a detection voltage VDET. The sense terminal CS of the controller 70 is coupled to the resistor 30 which serves as a current-sense resistor for converting a primary-side switching current IP into a current signal VCS. The controller 70 generates the switching signal VPWM at the pulse width modulation terminal VPWM for switching the transformer 10 in response to the detection voltage VDET at the detection terminal DET and the current signal VCS at the sense terminal CS.
Furthermore, the voltage-compensation terminal COMV and the current-compensation terminal COMI of the controller 70 are respectively coupled to the capacitors 31 and 32 for voltage-loop frequency compensation and current-loop frequency compensation to improve the stability of the power supply. The controller 70 therefore needs two extra pins for coupling the external capacitors 31 and 32. Since the capacitors 31 and 32 are bulk capacitors, they are inappropriate to integrate within the controller 70. The main problem that the power supply manufacturers have to solve is how to integrate the external capacitors 31 and 32 within the controller 70 with a smaller layout space, which saves the pin counts of the controller 70 and therefore reduces the packaging cost thereof.
Bulk capacitors, such as capacitors 31 and 32 mentioned above, are necessary components for low-pass filters which are utilized for frequency compensation. Nowadays, although various types of low-pass filters are able to integrate within the controller chip, they still failed to meet low bandwidth requirement of power supplies. This is because most of their bandwidths are too high due to the consideration of saving layout space.
Referring to FIG. 2, it shows a circuit diagram of conventional switched-capacitor low-pass filter. The conventional switched-capacitor low-pass filter comprises a first capacitor 75, a second capacitor 76, a first switch 77, and a second switch 78. The first switch 77 is coupled in between an input terminal IN of the low-pass filter and the first capacitor 75. The second switch 78 is coupled in between the first capacitor 75 and the second capacitor 76. The second capacitor 76 is coupled to an output terminal OUT of the switched-capacitor low-pass filter. The input terminal IN of the low-pass filter receives an input signal SIN. The first capacitor 75 and the second capacitor 76 filter the input signal SIN for generating an output signal SOUT1 at the output terminal OUT of the low-pass filter.
Referring to FIG. 7, it shows the waveforms of the input signal SIN and the output signal SOUT1 shown in FIG. 2. The capacitance of the second capacitor 76 of the low-pass filter, for example, is twenty times that of the first capacitor 75. The low-pass filter shown in FIG. 2 filters the input signal SIN to generate the output signal SOUT1. As shown in FIG. 7, the difference between the waveform of the input signal SIN and the waveform of the output signal SOUT1 is small. That is, the bandwidth of the low-pass filter is not low enough, and therefore the medium-high frequency harmonic of the input signal SIN cannot be filtered efficiently. In order to improve the filtering efficiency of the low-pass filter shown in FIG. 2, when the frequency to switch the capacitor is kept constant, the ratio of the capacitance of the second capacitor 76 to the capacitance of the first capacitor 75 must be increased. The integrated layout space of the second capacitor 76 will therefore inevitably increase, which also increases the integration cost of the controller chip.
In addition to the low-pass filter described above, there are also other types of low-pass filters. For example, U.S. Pat. No. 7,049,883 (Makino et al.) disclosed an active low-pass filter which cannot apply a smaller capacitance of capacitor to meet the low bandwidth requirement of the controller 70 of the power supply.
The present invention provides a low-pass filter solution to the problems described above, which utilizes capacitors having smaller capacitance to achieve the same low-pass filtering performance as the external compensation capacitor having bulk capacitance does in conventional techniques. The present invention further facilitates the integration of the controller chip and reduces the prime cost enormously.